In a typical welding method, many objects tend to expand when heat is applied to a welded portion and the like. In this manner, the heat serves a similar function to an external force that deforms an object. Here, when an object is uniformly heated, the object freely expands and is isotropically deformed. However, in a case where only a portion to be welded is locally heated like welding, the expansion is obstructed by surrounding materials, and this causes thermal stress in the object and brings about residual stress.
Usually, only the vicinity of a welded portion is heated/cooled in a steel welded portion. However, thermal deformation of expansion/constriction caused thereby is restricted by a surrounding base material, and yield stress of a steel material decreases with temperature, and plastic deformation of compression is caused to the material in the middle of heating.
Only the welded portion (in a case of a rod model) becomes short after being cooled due to a plastic strain of the compression. However, because the welded portion and the base material have been joined, the welded portion is stretched by the surrounding base material. As a result of that, tensile residual stress is caused in the vicinity of the steel welded portion.
Patent Literature 1 discloses a joining method using resistance welding regarding high pressure hydraulic piping and a joining method thereof, and Patent Literature 2 discloses that projection welding is used for a welded structure of a shaft member and a plate member.
FIG. 11 is a residual stress distribution diagram of typical fused joining. In this fused joining, distribution of tensile residual stress can be observed over a large area around a joined portion. Such tensile residual stress in the joined portion has a major defect of reducing the fatigue strength although depending on the extent.
Further, because the welded portion may have various defects including a texture defect such as a blowhole and a shape defect such as a notch, it is indispensable to conduct a qualification test such as X-ray inspection and ultrasonic testing for important welded parts.
Under such circumstances, it is necessary to design a product in consideration of a large safety factor for an important product, or to design a product not to include a welded portion. In any case, it is the fact that, in manufacturing machine products and automobile parts, great attention has been given to the welded portion therein not only to the product design but also to quality assurance in a manufacturing department.
Also, conventionally, quenching heats a metal material to high temperature, and rapidly cools the material in water or in oil. In a case of steel, the steel is heated to be an austenite structure and is then rapidly cooled, thereby being changed into a martensitic structure, and being extremely hardened.
To obtain the martensitic structure by quenching the steel, it is necessary to have a carbon equivalent above a certain level, and to cool the steel at a critical cooling rate or more.
As a method of enhancing the fatigue strength of a steel product, a method of thermal treatment such as carburization quenching, high-frequency quenching, and a soft-nitriding method has been known in which martensitic transformation is caused on a surface and compressive residual stress is generated on the surface. However, such heat treatment cannot be used for a welded part. This is because a weld defect may be included in a welded portion. Therefore, there is a concern that hardening of the surface of the welded portion may further cause a decrease in reliability of the welded portion. Therefore, in a case where parts that include a welded portion are subjected to a carburization process, it is typical to perform anti-carburization before the carburization process in order to prevent the welded portion from being carburized.
Here, the applicant of the present patent application previously disclosed a press-fit joining method in Patent Literature 3. In this method, a shaft member is solid-phase joined with a plate member (an opening portion is drilled) by means of press-fit joining using a jig such as an electrode.